It is of general interest and concern to reduce distortion in radio frequency communication, which is usually in channels as close together in frequency as practicable in order to maximize use of radio frequency spectrum available to communications concerned. Non-linearity in transmitter mixer and output amplifier stages operative for one channel can lead to unwanted emissions in nearby channels. Many techniques of considerable sophistication have been used or proposed for use in reducing distortion. Such techniques include separate processing of in-phase and quadrature phase components (I/Q) of normally analogue RF signals, and digital signal processing (DSP) in which analogue RF signals are passed through an analogue-to-digital converter for processing digitally in special-purpose logic hardware or program-controlled processing or arithmetic unit with results often converted back from digital to analogue form. Both of I/Q and DSP techniques have inherent capability to correct both amplitude and phase errors. However, they tend to be rather complex and directed at particular radio communication systems, also quite costly to implement.